The present disclosure relates to a thermal transfer sheet, particularly, to a component forming a heat resistant smooth layer provided in the thermal transfer sheet.
A thermal transfer method using a sublimation dye is employed for reproducing a full-color image by color dots of a multiple of colors by transferring the multiple of color dots onto a material to which the color dots are to be transferred by heating for a remarkably short time. In the thermal transfer method, a so-called sublimation type thermal transfer sheet in which a dye layer including the sublimation dye and a binder is provided on one surface of a base sheet such as a polyester film is used as a thermal transfer sheet.
Further, in the thermal transfer method, the thermal transfer sheet is heated from a back thereof by a heating unit such as a thermal head according to image information, and an image is formed by transferring a dye contained in the dye layer onto a material (printing paper) to which the dye is to be transferred. It is necessary that a friction between a surface of the thermal transfer sheet contacting the thermal head and the thermal head is stably low from a low density printing to a high density printing. Therefore, in the thermal transfer sheet in general, a heat resistant smooth layer is provided on a surface which is the other side of the surface on which the dye layer is formed for the purposes of preventing fusion with the thermal head and imparting good running smoothness (smoothing property, lubricity).
By the way, in the case of printing on a printing paper by using the thermal transfer sheet, heat is imparted to the heat resistant smooth layer from the thermal head to transfer the dye in the dye layer on the reverse surface onto the printing paper. A color optical density is proportional to heat quantity imparted from the heating unit such as the thermal head, and a surface temperature of the heating unit such as the thermal head is changed by the unit of a several hundreds of degrees (generally from an ordinary temperature to about 250° C.). Therefore, a friction coefficient between the thermal head and the heat resistant smooth layer is easily changed by a temperature change when the thermal transfer sheet moves on the thermal head. When the friction coefficient between the thermal head and the heat resistant smooth layer is changed, it is difficult for the thermal transfer sheet to move at a constant speed, and, therefore, it is difficult to obtain a clear image. More specifically, the movement of the thermal transfer sheet is temporarily slowed down when the friction coefficient is large, and a density of the slowed portion is increased to cause so-called sticking (line-like printing irregularity) or the like.
In order to prevent the sticking, it is particularly necessary to reduce the friction coefficient at high temperatures. For example, use of phosphoric acid ester or fatty acid ester as a lubricant (smoothness imparting agent) and inclusion of phosphoric acid ester or fatty acid ester in the heat resistant smooth layer have been proposed to reduce the friction coefficient at high temperatures (for example, see Japanese Patent Application Laid-Open No. 10-35122).